1. Field of the Invention
This invention relates generally to a method and an apparatus for driving and controlling a thermal head used in a printing device such as a tape printer for printing on a tape recording medium. More particularly, the present invention relates to a method and an apparatus for properly driving and controlling a thermal head in response to the temperature variations of the environment and the thermal head.
2. Description of the Related Art
In recent years, printing devices which print on tape recording media have become very popular. Such tape recording media have on their backs an adhesive layer which is covered with peel-off tape. After printing, the paper is peeled off and the tape is affixed to a desired place such as a label. Since this kind of printing device (referred to as the tape printer in the present specification) must be small and compact, the printing mechanism used in the printer must also be small. A typical tape printer employs a thermal transfer printing mechanism including a thermal head.
When the thermal head is in use, the power provided to each heating element must be adjusted accorded to the temperature of the thermal head. The methods for adjusting the power are disclosed in the following Japanese patent laid-open publications:
Japanese Patent Laid-Open Publication SHO 62-121072 discloses a method for controlling the pulse width applied to the thermal head in correspondence with the radiating plate temperature which is measured on every printing operation.
Japanese Patent Laid-Open Publication SHO 64-14055 discloses a method for controlling the thermal head drive by measuring the temperature of a thermistor placed near the thermal head and predicting the temperature change of the thermal head.
Japanese Patent Laid-Open Publication HEI 2-2030 discloses a method in which printing is temporarily suspended if the temperature of the thermal head, when printing ends, is significantly different from that when printing started.
Japanese Patent Laid-Open Publication HEI 2-9649 discloses a method in which the conditions for suspending the head drive change according to the rate of the temperature change of the unit on which the thermal head is mounted.
Japanese Patent Laid-Open Publication HEI 2-25345 discloses a method for driving the thermal head by obtaining the temperature gradient near the thermal head and calculating the temperature of the thermal head from this temperature gradient.
Japanese Patent Laid-Open Publication HEI 2-45182 discloses a method for detecting cooling fan anomaly (overheat) based on the initial temperature and the temperature during printing of the thermal head. In thermal head printing, the condition of the ink ribbon used for thermal transfer changes according to the temperature of the printing device environment. In other words, the quality of the printed dots changes. Therefore, in order to maintain a good print quality, the heat generation of each heating element of the thermal head must be controlled in conjunction with the temperature of the printing device environment.
Japanese Patent Laid-Open Publication SHO 62-23767 discloses a method for controlling the thermal head drive using one sensor for measuring the temperature of the thermal head and another for measuring the ambient temperature.
Japanese Patent Laid-Open Publication HEI 2-121853 discloses a method for controlling the thermal head drive in which the ambient temperature is measured every time the initial setting of the printer is made and then compared with the previous measurement. The driving power provided to the thermal head is calculated according to a predetermined computation procedure. Then, the thermal head is driven according to the result of the computation.
In Japanese Patent Laid-Open Publication SHO 62-23767 above, two temperature sensors are required. Hence this method is not appropriate to a tape printer which must be small and compact as mentioned above.
If a temperature sensor such as a thermistor is placed near the thermal head in Japanese Patent Laid-Open Publication HEI 2-121853 above, the sensor may not be able to measure the temperature of the printing device environment accurately because of the heat generated by the thermal head. This may make it difficult to control the thermal head drive. The reason is that the thermal head becomes a heat source when it operates. Therefore, the temperature measured with the thermistor installed near the thermal head may be quite different from the actual temperature of the printing device environment. For example, even if the ambient temperature does not change, the temperature measured before the thermal head begins operating is different from that measured after printing is completed. Thus, it is not possible to control the thermal head drive properly according to the environmental temperature alone.
The present invention intends to overcome the problems described above.